The term "full duplex" in the context of a communication device means that the device simultaneously transmits and receives signals. To the user, this means that he or she can simultaneously talk and listen to another party through the device. Conversely, the term "half duplex" in this context means that the device can only transmit or receive at one time, but not both. Full duplex is obviously better than half duplex communication because it enables parties to communicate from remote locations as if they were standing face to face. However, full duplex communication is more difficult to implement in speaker phones because of the problem of acoustical and electrical feedback. Acoustical feedback occurs when sound from the speaker travels back to the microphone. Electrical feedback is similar, yet pertains to the electrical signals representing the audio input (the signal transmitted to the remote device) and the audio output (the signal received from the remote source). Electrical feedback occurs when the transmit and receive circuits are not completely isolated from each other and form a closed loop with a loop gain greater than one. To eliminate feedback entirely, the overall loop gain, including both acoustical and electrical effects, must be less than one.
The majority of speaker phones for hands-free and group communication are half duplex configurations that utilize fast switching circuitry to alternate between: 1) broadcasting audio output through a speaker, and 2) listening for audio input in a separate microphone. If this switching did not take place, the speaker would produce an annoying squeal due to the acoustical feedback path from the speaker to the microphone. The switching circuitry prevents the speaker and microphone from being active at the same time, and therefore, audio output from the speaker will not induce electrical signals in the microphone. While the switching avoids the squeal, it can be annoying in itself because the user cannot speak and listen at the same time. The switching circuitry compares the strength of the broadcast signals from each direction and allows only the stronger of the two to be transmitted to the opposite end.
Since full duplex communication requires a complete closed loop for simultaneously sending and receiving signals between the two locations of conversation, the overall loop gain must be less than one. One way to ensure that the loop gain is less than one is to use digital signal processing to detect feedback and attempt to cancel it. Speaker phones that employ this approach are sometimes referred to as digital full duplex speaker phones. These devices include a separate speaker and microphone, and an analog to digital conversion circuit to recognize, with adaptive filters, the signal gain variances between the transmit and receive signal paths caused by audio output of the receive signal entering the transmit portion of the loop via the microphone. In response to detecting this feedback, these devices use electronically controlled attenuators in each side of the loop to ensure that the loop gain is less than one.
While the attenuators can reduce the annoying squeal of feedback, they can tend to undermine performance of the device by reducing the gain on the audio output to such an extent that is difficult for the user to hear. At times, the attenuator needs to reduce the gain on the receive signal so much that the user cannot here the other party's voice. In addition to this drawback, the digital configurations are several times more costly then the half duplex speaker phone configurations.
One proposed solution to the acoustical feedback problem is shown in U.S. Pat. No. 4,002,860, which describes a communication device that uses a single transducer as both a speaker and microphone to eliminate acoustical feedback. The circuit design shown in this patent does not effectively eliminate electrical feedback however. This circuit uses devices called hybrid transformers in an attempt to isolate the transmit and receive signals from each other. The telephone circuits used to provide isolation in this circuit are actually only capable of providing about 15 dB of isolation. The amount of isolation is also highly dependent on the degree to which the circuit can match the impedance of the transducer and of the telephone line. Because of the circuit's inability to isolate the transmit and receive signals, it will generate a significant amount of feedback for this reason alone.
Another drawback of the circuit shown in the '860 patent is that the circuit applies a load across the transducer. Loading of the transducer can significantly undermine the effectiveness of this circuit because it interferes with the transmit signal induced in the transducer from an acoustical voice input. In a typical microphone, the signals induced from the user's voice are quite small on the order of 10 mV. Any loading of the transducer draws away the energy of the induced signal. To deal with these losses, the circuitry for processing the transmit signal can amplify the small voice signals, but if there is any feedback of the receive signal to the amplifier in the transmit circuitry, the feedback problem highlighted above becomes even worse.